1. Field of the Invention
The present invention relates generally to conversion of analog signals to digital signals and, more particularly to both predictive and error feedback coders that sample the analog signal at generally high rates and provide a differential pulse code modulation (DPCM) output.
2. Description of the Prior Art
Sophisticated digital integrated circuit technologies have made digital signal processors which perform a variety of functions a practical alternative to conventional analog systems. The trend toward digital processing is accompanied by many advantages, including precisely predictable performance, the ability to share hardware over several separate channels without interchannel distortion, and the flexibility of programming one processor to perform a variety of functions.
Since digital processors must typically interface with continuous signals, analog/digital and digital/analog converters thus become increasingly important system components. Many such converters have been developed, including predictive coders which reduce the dynamic range of the input signal applied to the quantizer within the coder, or error feedback coders which shape the spectral distribution of the quantizing error so as to reduce in-band noise. Both types of coders may make use of tapped delay lines in the feedback path, with the n outputs of the delay line being fed through n separate attenuators having coefficients a.sub.1 . . . a.sub.n, and then summed. With this arrangement, various techniques are available for choosing appropriate attenuator coefficients. In each technique, it is desirable to minimize the number of required quantizaton levels, so that the number of bits needed to adequately represent the signal being encoded is reduced. Certain of the prior art techniques for coefficient optimization have been developed primarily for transmission systems. However, in such systems, oversampling, as desired in the present invention, cannot be used, since it results in an undesirably high transmission rate. Accordingly, one subject of the present invention is to optimize the coefficient selection technique in cases where oversampling is acceptable.
Other prior art techniques known in the field of video transmission have utilized moderate oversampling in conjunction with single or double integration feedback networks. However, these systems have also been limited in the degree of quantizer simplification achieved. Accordingly, another object of the present invention is to retain the signal independent feedback network as in video coders, while achieving a much greater reduction in the number of required quantization levels.